In recent years, various kinds of transfer material, such as glossy paper, cards, postcards and the like have come to be used, other than ordinary paper or film for overhead projectors (OHP), as transfer materials in full-color printers, full color copying machines and the like. Transfer methods that use intermediate transfer members have become therefore a mainstream feature.
In transfer methods that use intermediate transfer members, ordinarily, a visible toner image is transferred from an image bearing member to an intermediate transfer member, after which the toner image must be transferred again from the intermediate transfer member onto a transfer material. The number of transfers is thus greater than in conventional methods, and hence there is a demand for toners having higher transfer efficiency.
Ways of enhancing the transfer efficiency of toner include, for instance, spherization, through heating of toner that has been produced through pulverization (hereafter also referred to as pulverized toner). In a spherization process of pulverized toner through heating, wax leaches readily at the toner surface, and the surface abundance of wax may increase accordingly. This may result in decreased fluidity, worse fogging on account of lower charge quantity, and may give rise to melt adhesion of toner onto the image bearing member.
Patent document 1 discloses a toner that can be obtained by causing toner base particles to adhere to an external additive, and subjecting the toner base particles, in a dispersed state, to a surface modification treatment by hot air. This toner exhibits high-fluidity and high-charging performance characteristics, but may exhibit fogging at non-image portions, since it is difficult to completely remove toner in a process (cleaning process) in which there is removed residual toner that remains after transfer. In high-speed printing, such as print on demand (POD), moreover, the density of the obtained images fluctuates significantly. Charging stability of the toner has thus room for improvement.
Patent document 2 discloses a toner obtained by causing two types of external additive, having dissimilar average particle sizes, to adhere to toner base particles, and by performing a surface modification treatment by hot air with the toner base particles in a dispersed state.
Patent document 3 discloses a toner obtained by adding silica having an average primary particle size ranging from 35 to 300 nm and silica having an average primary particle size ranging from 4 to 30 nm, followed by spherization by a heat treatment.
The toners disclosed in Patent documents 2 and 3 have some degree of durability against toner stress in a developing device. However, in a case where such toner is used as a two-component developer in high-speed machines, such as in POD, there varies the triboelectric charge quantity with a magnetic carrier. This gives rise to changes in image density and fogging in non-image portions. Also, the fluidity of the developer in the developing device may become impaired. Low-temperature fixability and image gloss may likewise become impaired, and the adhesion force onto a fixing member may increase, causing the paper to wrap around the fixing unit as a result.
Thus, the toners disclosed in Patent documents 1 to 3 are not found to be satisfactory enough, being in need for further improvement, as regards charging stability, low-temperature fixability, image gloss, and fixing wrapping resistance in cases where the toner is used in high-speed machines, such as in POD.